The present invention relates to an optical disc apparatus reading an information recorded on a recording surface of an optical disc and recording an information, and an optical pickup and an objective lens driving apparatus used therefore.
In the optical disc apparatus, in order to make recording and reproducing speeds of the information high, there has been known a method of making a rotating speed of the optical disc high. In accordance that the optical disc rotates at a higher speed, it is necessary to increase a driving force generated in a focusing coil and a tracking coil in order to make the objective lens driving apparatus to track the optical disc rotating at a high speed.
Accordingly, in order to increase the driving force of the objective lens driving apparatus, for example, in an optical disc apparatus described in patent document 1 (JP-A-2003-168230 (page 9, FIG. 9)), two branch yokes branched from a magnetic yoke are provided in a standing manner so as to be extended between focus coils. Therefore, an efficiency of a magnetic circuit is increased by increasing a rate of a coil portion generating an effective driving force with respect to an entire length of the coil.
In the conventional optical disc apparatus described in the patent document 1, since the branch yokes are arranged in an inner side of a bottom plate portion of the magnetic yoke, it is impossible to increase a height of the branch yokes in the case that the branch yokes are manufactured by being cut up from the bottom plate portion of the magnetic yoke. In other words, since the cut-up portion is manufactured in such a manner that the heights of the branch yokes are uniform while taking an uniformity of the magnetic circuit into consideration, it is unavoidable that the height of the branch yoke is equal to or less than one half of the height of the cut-up portion. In order to solve the problem, a thickness of the bottom plate portion is reduced by extending the bottom plate portion forming the branch yokes, whereby the height of the branch yokes is increased.
However, in accordance with these methods, in the case that the height of the branch yoke is reduced, a distribution of magnetic flux density becomes non-uniform in a height direction, and a dispersion of a driving force driving the objective lens driving apparatus is increased. Further, if the branch yoke becomes thin, a cross sectional area of a magnetic path becomes small, an efficiency of a magnetic circuit is lowered, and a complex working step is required. As a result, it is impossible to increase the driving force of the objective lens driving apparatus on the basis of any method.
On the contrary, if the branch yokes are cut up from an outer edge of the bottom plate portion of the magnetic yoke, the height and the thickness of the branch yokes can be set to predetermined values. However, since it is necessary to guide a laser light to an objective lens in the objective lens driving apparatus, an opening at least equal to or larger than a light flux of the light guided to the object lens is necessary in the bottom plate portion of the magnetic yoke. Since a width of the bottom plate portion from the opening to the branch yoke is limited, a cross sectional area of the magnetic path is lowered.